Hepatitis A virus has been successfully adapted to growth in African green monkey kidney tissue culture. The virus is predominantly cell-associated and does not produce cytopathic effects (CPE). It was attenuated for chimpanzees after 10 or 20 serial tissue culture passages. The attenuated virus has been shown not to revert to virulence during serial passage through chimpanzees. The tissue culture-adapted HAV has been thrice cloned and clones representing passages 20 and 30 have been characterized in chimpanzees and marmosets and compared with wild-type virus. Immunization of chimpanzees with the attenuated virus protected most but not all vaccinated animals.